Reference is made to my book entitled INTERNAL REFLECTION SPECTROSCOPY, published 1967 by Interscience Publishers. This book systematically describes theory, instrumentation and applications of internal reflection spectroscopy (IRS), whose contents are hereby incorporated by reference. Chapter 4 describes in detail various geometries of internal reflection elements (IRE), which essentially consist of a transparent optical element for the radiation involved having an entrance or first surface for receiving a focussed or collimated beam of incident radiation of an angle that allows the beam to enter the element and that causes the beam to become incident on a second or sampling surface containing the sample to be investigated at an angle exceeding the critical angle so that total reflection occurs and the resultant beam is caused to exit the element at a third surface and thereafter can be optically processed in a standard spectrometer. As is explained in the book, at the sampling surface the beam becomes modulated by interaction of its evanescent wave, usually by absorption, with the sample, so that when the modulation content of the existing beam is transformed into a curve of absorption as a function of beam wavelength, the usual absorption spectrum of the sample is obtained.
IRS has a number of advantages over other spectroscopy techniques. It can be used with liquid or solid samples and little or no sample preparation is required. This is especially important for very small samples, e.g., microgram and nanogram quantities, for which there is currently very wide interest. Whenever the internal reflection method can be used to obtain an effective pathlength greater than the actual thickness of a (film) sample, then an improvement in spectral contrast, thus sensitivity, is achieved relative to simple transmission. Under appropriate conditions, this increase in sensitivity may be as high as ten. Another important advantage of IRS in micro- and nano-sampling applications is the ease with which samples are prepared and handled. It is only necessary to bring the sample in contact with or in close proximity to the sampling surface. For example, to record the spectrum of a fiber, one needs only to place the end of the fiber in contact with a suitable IRE to record its spectrum.
Because of the above-mentioned advantages, internal reflection is the preferred method for micro- and nano-sampling. In principle, such experiments could be done using a large prism (hemisphere) and diffraction limited optics, but there would exist the problems of locating the sample on the micron-size sensitive area. Hence, for such small samples, the IRE or prism size should be chosen to be comparable to that of the sample. However, such small prisms cannot be easily fabricated or handled. If larger prisms are used, means must be devised for masking the light beam to make its focus comparable to the size of the sample and finally to place the sample on the sampling surface at the exact location from which the small light beam is reflected. A further disadvantage of the larger prism is that the light beam is refracted as it enters the prism and the beam focus size is larger at the sampling surface; hence the light intensity and therefore sensitivity is reduced.
In my book I describe a technique for conducting investigations of minute samples. But the techniques described require multiple reflections at the sampling surface, with the result that only a small fraction of the radiation interacts with the sample and thus the degree of modulation of the exiting beam is extremely small making for poor spectra. Thus, the techniques described therein have not been completely satisfactory.